Shift Work and Cancer: State of Science and Practical Consequences

Relationship between Night Shifts and Risk of Breast Cancer among Nurses: A Systematic Review

Background and objectives: The incidence of breast cancer worldwide has increased in recent decades, accounting for 1 in 3 neoplasms in women. Besides, nurses are mainly represented by the female collective, most of them, undertaking working conditions with intensive rotative and night shifts due to the 24-h pace of work of this profession. The objective of this study was to assess the possible relationship between shift work, especially night-time work, and the development of breast cancer among nurses. Materials and Methods: A systematic review of the literature was carried out through the consultation of the following databases: Cochrane Plus Library, PubMed, CINAHL (Cumulative Index to Nursing and Allied Health Literature), Web of Science, ScienceDirect, Scopus, and Dialnet. Records were selected between 2010 and 2020, in Spanish and English, which covered the association between breast cancer diagnosed among nursing professionals and rotating night shifts. Results: A total of 12 studies were identified after critical reading. Most of the studies found an association between breast cancer and consecutive rotating night shifts prolonged over time. Among the associated factors, the alteration of the circadian rhythm influenced the expression of peripheral clock genes, which was the same as reproductive hormones. The risk of breast cancer in nurses increased during early adulthood and after 5 or more years with 6 or more consecutive nights. Conclusions: The different studies of this review show significant associations between breast cancer and prolonged rotating night shifts. Similarly, there is a relationship between the alterations in certain circadian rhythm markers (such as melatonin), epigenetic markers (such as telomeres), and breast cancer that would require more studies in order to corroborate these findings.

Occupational Risk Factors for Prostate Cancer: A Meta-analysis

Prostate cancer is the second most common cancer in men worldwide. There are many occupational factors that have been suggested to cause prostate cancer. Our aim was to evaluate the evidence for causality by a literature review of occupational factors. We searched literature in Medline and SCOPUS from 1966 to June 30, 2015 to identify occupational risk factors for prostate cancer. The following risk factors were selected: farmers/agricultural workers, pesticides - whole group, and separately organophosphate and organochlorine pesticides, carbamates and triazines, cadmium, chromium, cutting fluids, acrylonitrile, rubber manufacturing, whole body vibration, shift work, flight personnel, ionizing radiation, and occupational physical activity. For each factor a literature search was performed and presented as meta-analysis of relative risk and heterogeneity (Q and I2 index). A total of 168 original studies met the inclusion criteria with 90,688 prostate cancer cases. Significantly increased risks were observed for the following occupational exposures: pesticides (metaRR = 1.15, 95% confidence interval [CI] = 1.01-1.32; I2 = 84%), and specifically group of organochlorine pesticides (meta relative risk [metaRR] = 1.08, 95% CI = 1.03-1.14; I2 = 0%), chromium (metaRR = 1.19, 95% CI = 1.07-1.34; I2 = 31%), shift work (metaRR = 1.25, 95% CI = 1.05-1.49; I2 = 78%) and pilots (metaRR = 1.41, 95% CI = 1.02-1.94; I2 = 63%) and occupational physical activity in cohort studies (metaRR = 0.87, 95% CI = 0.81-0.94; I2 = 0%). The literature review supports a causal association for a few of the previously suggested factors.

The Pathophysiologic Role of Disrupted Circadian and Neuroendocrine Rhythms in Breast Carcinogenesis

Most physiological processes in the brain and body exhibit daily (circadian) rhythms coordinated by an endogenous master clock located in the suprachiasmatic nucleus of the hypothalamus that are essential for normal health and functioning. Exposure to sunlight during the day and darkness at night optimally entrains biological rhythms to promote homeostasis and human health. Unfortunately, a major consequence of the modern lifestyle is increased exposure to sun-free environments during the day and artificial lighting at night. Additionally, behavioral disruptions to circadian rhythms (ie, repeated transmeridian flights, night or rotating shift work, or sleep disturbances) have a profound influence on health and have been linked to a number of pathological conditions, including endocrine-dependent cancers. Specifically, night shift work has been identified as a significant risk factor for breast cancer in industrialized countries. Several mechanisms have been proposed by which shift work-induced circadian disruptions promote cancer. In this review, we examine the importance of the brain-body link through which circadian disruptions contribute to endocrine-dependent diseases, including breast carcinogenesis, by negatively impacting neuroendocrine and neuroimmune cells, and we consider preventive measures directed at maximizing circadian health.

Circadian variation of the human metabolome captured by real-time breath analysis

Circadian clocks play a significant role in the correct timing of physiological metabolism, and clock disruption might lead to pathological changes of metabolism. One interesting method to assess the current state of metabolism is metabolomics. Metabolomics tries to capture the entirety of small molecules, i.e. the building blocks of metabolism, in a given matrix, such as blood, saliva or urine. Using mass spectrometric approaches we and others have shown that a significant portion of the human metabolome in saliva and blood exhibits circadian modulation; independent of food intake or sleep/wake rhythms. Recent advances in mass spectrometry techniques have introduced completely non-invasive breathprinting; a method to instantaneously assess small metabolites in human breath. In this proof-of-principle study, we extend these findings about the impact of circadian clocks on metabolomics to exhaled breath. As previously established, our method allows for real-time analysis of a rich matrix during frequent non-invasive sampling. We sampled the breath of three healthy, non-smoking human volunteers in hourly intervals for 24 hours during total sleep deprivation, and found 111 features in the breath of all individuals, 36–49% of which showed significant circadian variation in at least one individual. Our data suggest that real-time mass spectrometric "breathprinting" has high potential to become a useful tool to understand circadian metabolism, and develop new biomarkers to easily and in real-time assess circadian clock phase and function in experimental and clinical settings.

The effects of shift work on sleeping quality, hypertension and diabetes in retired workers

BACKGROUND

Shift work has been associated with adverse health effects by disturbing circadian rhythms. However,its potential long-term health effects and the persistent effects after leaving shifts have not been well established.

METHODS AND RESULTS

We studied 26,463 workers from Tongji-Dongfeng Cohort in China. All the participants are retired employees of Dongfeng Motor Company. Information on demographics, occupational history and medical history were gathered through questionnaires. After adjusting potential confounders in the logistic regression models, shift work was associated with poor sleeping quality, diabetes and hypertension independently. We observed significant effects of shift work on poor sleeping quality, diabetes and hypertension; the ORs (95%CI) are 1.18 (1.09-1.27), 1.10 (1.03-1.17) and 1.05 (1.01-1.09) respectively. In the further analysis, we found elevated ORs (95%CI) for participants with poor sleeping quality, the ORs (95%CI) are 1.34 (1.08-1.60), 1.13 (1.05-1.21), 1.05 (1.03-1.07) and 1.05 (1.01-1.09) for 1-4, 5-9, 10-19, ≥20 years of shift work respectively. However, with the extension of leaving shift work duration, the effects of shift work on sleep quality gradually reduced.

CONCLUSIONS

Shift work may be an independent risk factor for sleeping quality, diabetes and hypertension even in retired workers. Applicable intervention strategies are needed for prevention of sleep loss, diabetes, and hypertension for shift workers.

Does night work increase the risk of breast cancer? A systematic review and meta-analysis of epidemiological studies

OBJECTIVE

To conduct a systematic review, with meta-analysis, of studies assessing the association between night work and the risk of breast cancer, using available epidemiological evidence.

METHOD

Relevant studies were identified by searching several databases and the reference lists of retrieved articles. We combined the relative risks (RR) from individual studies using a random-effects model. Subgroup analysis was carried out as the data showed statistically significant heterogeneity.

RESULTS

Thirteen studies consisting of eight case-control studies and five cohort studies were included in the analysis. In the combined analysis of all studies, night work was associated with an increased risk for breast cancer (RR = 1.20, 95%CI = 1.08-1.33). The higher-quality studies showed a similar finding with a pooled RR of 1.40 (95%CI = 1.13-1.73). Both case-control studies (RR = 1.32, 95%CI = 1.17-1.50) and cohort studies (RR=1.08, 95%CI = 0.97-1.21) showed a positive association between night work and the risk of breast cancer. No publication bias was found either from Begg's funnel plot (P = 0.086) or the Egger's test (P = 0.107). Additional well-conducted and large-scale epidemiological studies are needed.

A retrospective cohort study of shift work and risk of cancer-specific mortality in German male chemical workers

OBJECTIVES

Human evidence of carcinogenicity concerning shift work is inconsistent. In a previous study, we observed no elevated risk of total mortality in shift workers followed up until the end of 2006. The present study aimed to investigate cancer-specific mortality, relative to shift work.

METHODS

The cohort consisted of male production workers (14,038 shift work and 17,105 day work), employed at BASF Ludwigshafen for at least 1 year between 1995 and 2005. Vital status was followed from 2000 to 2009. Cause-specific mortality was obtained from death certificates. Exposure to shift work was measured both as a dichotomous and continuous variable. While lifetime job history was not available, job duration in the company was derived from personal data, which was then categorized at the quartiles. Cox proportional hazard model was used to adjust for potential confounders, in which job duration was treated as a time-dependent covariate.

RESULTS

Between 2000 and 2009, there were 513 and 549 deaths among rotating shift and day work employees, respectively. Risks of total and cancer-specific mortalities were marginally lower among shift workers when taking age at entry and job level into consideration and were statistically significantly lower when cigarette smoking, alcohol intake, job duration, and chronic disease prevalence at entry to follow-up were included as explanatory factors. With respect to mortality risks in relation to exposure duration, no increased risks were found in any of the exposure groups after full adjustment and there was no apparent trend suggesting an exposure-response relation with duration of shift work.

CONCLUSIONS

The present analysis extends and confirms our previous finding of no excess risk of mortality associated with work in the shift system employed at BASF Ludwigshafen. More specifically, there is also no indication of an increased risk of mortality due to cancer.